A decade ago the idea of a pool of global professional services firms that counted just five among its number might have seemed questionable.

But even a few months ago, the concept of a Big Four appeared unthinkable.Today, thanks to economic Darwinism, it appears all but certain: even if the proposed tie-up between Andersen and KPMG unravels.

The fact that the discussions have reached a detailed stage has only been made possible by the exclusion of Andersen?s US practice from the deal: no merger partner would be willing to pursue a link that involves a national firm with a criminal indictment hanging over its head.

Quite simply, when things are as bad as they currently are for Andersen, only the fittest can survive.

Andersen senior UK partner John Ormerod might have sought to exude confidence at a hastily agreed press conference in London this morning, but a deal is by no means cut and dried.

Competition regulators at the Financial Services Authorities and the European Commission will take some convincing that a Big Five should become a Big Four. But what are the alternatives?

If the KPMG deal is ruled out on those grounds, a deal with Deloitte & Touche or Ernst & Young would never get the green light either.

And if no partner could be found Andersen would simply disintegrate. Many of its best partners and staff would be cherry picked by its rivals and the wounded firm would be left to limp on alone with a fatally damaged brand name.

A Big Five could only be maintained if Andersen were to reverse into a mid-tier firm like BDO Stoy Hayward, which has already been casting around for disaffected Androids.

But Ormerod doesn’t want that – he made that much clear at this morning’s press conference. And it’s hard to see how his colleagues would either.

If the cultural hurdles that had to be overcome in the merger of Price Waterhouse and Coopers and Lybrand seemed high, they may be insurmountable for a Big Five and mid-tier firm.

So a Big Four it is. Anyway you look at it.

But the KPMG deal itself is less secure. Yes, KPMG would be getting some talented people and some valuable clients but it will also be acquiring damaged goods – even if the US is cut adrift. In its indictment of Andersen for obstruction of justice in the US, the Justice Department implicated the London office.

Don’t underestimate the significance of that: Andersen’s biggest firm outside of the US is the UK, and most of that business is done out of London.

But the biggest obstacle might be at the KPMG end: if its clients and staff worry about the wisdom of the tie-up the firm might wothdraw.

It was notable that in the hours following this morning?s announcement, Ormerod and his senior colleagues set about telling clients and staff that the deal ‘chosen’ by the firm was a good one.

Over at KPMG, meanwhile, senior partner Mike Rake was busily writing to his clients and staff reassuring them that these discussions were not an act of commercial madness. As one KPMG staffer put it following the announcement: ‘This is just the beginning of a journey.’